1. Field of the Invention
This invention relates to the preparation of absorbent polymeric compositions by a procedure simpler and more economical than any heretofore known in the prior art.
2. Description of the Prior Art
Polymeric substances which have the ability to absorb aqueous fluids are known in the prior art. For example, U.S. Pat. Nos. 3,669,103 and 3,810,468 disclose that a variety of monomers may be polymerized, with crosslinking, to give polymeric absorbents. The crosslinking reaction is of critical importance, since the uncrosslinked polymers are water soluble and thus have no utility as absorbents.
Water-absorbing alkali metal salts of saponified granular starch-polyacrylonitrile (PAN) graft copolymers are disclosed in U.S. Pat. No. 3,661,815. In this disclosure, starch is graft polymerized in the granular state, and the saponification is carried out in an alcohol-containing medium to obtain a granular, insoluble material capable of absorbing about 100 times its weight of water. U.S. Pat. No. 3,932,322 discloses a mixture of the composition of U.S. Pat. No. 3,661,815 with fumed silica or alumina. This mixture exhibits an increased rate of fluid uptake and a decreased tendency toward dusting.
Water-absorbing alkali metal salts of saponified gelatinized starch-PAN graft copolymers are disclosed in U.S. Pat. No. 3,997,484, herein incorporated by reference. In this disclosure, starch is gelatinized by heating in water prior to graft polymerization; also, the graft copolymer is saponified in water to give a viscous dispersion of highly swollen but still insoluble microgel particles. Contrary to the absorbent composition of U.S. Pat. No. 3,661,815, the composition of U.S. Pat. No. 3,997,484 may be dried to a continuous film which has an unusually high absorbency for aqueous fluids. Moreover, this film-forming tendency permits a variety of substrates to be coated with thin films of the absorbent composition and thus leads to dramatic increases in fluid absorbencies of the substrates.